U.S. patent number 8,395,366 [Application Number 13/110,094] was granted by the patent office on 2013-03-12 for power factor correction converter including input current detecting circuit.
This patent grant is currently assigned to Murata Manufacturing Co., Ltd.. The grantee listed for this patent is Yoshiyuki Uno. Invention is credited to Yoshiyuki Uno.
United States Patent |
8,395,366 |
Uno |
March 12, 2013 |
Power factor correction converter including input current detecting
circuit
Abstract
A low-cost PFC converter capable of detecting an inductor
current including a DC component and performing appropriate
correction of a power factor with low loss includes a diode bridge
that rectifies an AC voltage input from an AC input power supply
Vac, a series circuit including an inductor and a switching
element, a rectifying and smoothing circuit that is connected in
parallel to the switching element and that includes a diode and a
smoothing capacitor, and a digital signal processing circuit that
performs on/off control on the switching element so that an input
current input from the AC input power supply Vac has a similar
waveform with respect to an AC voltage. A current flowing through
the inductor during an off period of the switching element is
detected using a current detecting resistor, and a decreased
voltage of the current detecting resistor is sampled at the middle
of the off period of the switching element, thereby detecting an
average value of the input current.
Inventors: |
Uno; Yoshiyuki (Nagaokakyo,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Uno; Yoshiyuki |
Nagaokakyo |
N/A |
JP |
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Assignee: |
Murata Manufacturing Co., Ltd.
(Kyoto, JP)
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Family
ID: |
42225536 |
Appl.
No.: |
13/110,094 |
Filed: |
May 18, 2011 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110216558 A1 |
Sep 8, 2011 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCT/JP2009/060215 |
Jun 4, 2009 |
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Foreign Application Priority Data
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Nov 25, 2008 [JP] |
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2008-299058 |
Apr 14, 2009 [JP] |
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2009-098362 |
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Current U.S.
Class: |
323/284;
363/21.09; 363/21.13; 323/283; 363/37 |
Current CPC
Class: |
H02M
1/4225 (20130101); H02M 1/0009 (20210501); Y02B
70/10 (20130101) |
Current International
Class: |
G05F
1/00 (20060101); H02M 3/335 (20060101); H02M
5/45 (20060101); H02M 5/458 (20060101) |
Field of
Search: |
;363/21.12-21.14,21.17,34,37 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 637 789 |
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Feb 1995 |
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EP |
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1 198 058 |
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Apr 2002 |
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EP |
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2-168864 |
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Jun 1990 |
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JP |
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06-074088 |
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Oct 1994 |
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JP |
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07-177746 |
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Jul 1995 |
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JP |
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09-140144 |
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May 1997 |
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JP |
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2000-232789 |
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Aug 2000 |
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JP |
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2001-286149 |
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Oct 2001 |
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JP |
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2002-203988 |
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Jul 2002 |
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JP |
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2004-282958 |
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Oct 2004 |
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JP |
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2004-297943 |
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Oct 2004 |
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JP |
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2005-020994 |
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Jan 2005 |
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JP |
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2005-198370 |
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Jul 2005 |
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JP |
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2007-195282 |
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Aug 2007 |
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JP |
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2007-288892 |
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Nov 2007 |
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JP |
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Other References
Official Communication issued in International Patent Application
No. PCT/JP2009/060215, mailed on Sep. 8, 2009. cited by
applicant.
|
Primary Examiner: Berhane; Adolf
Assistant Examiner: Nash; Gary
Attorney, Agent or Firm: Keating & Bennett, LLP
Claims
What is claimed is:
1. A PFC converter comprising: a rectifier circuit arranged to
rectify an AC voltage input from an AC input power supply; a series
circuit connected in a subsequent stage of the rectifier circuit
and including an inductor and a switching element; a rectifying and
smoothing circuit connected in parallel to the switching element; a
switching control circuit arranged to perform on/off control on the
switching element so that an average value of an input current
input from the AC input power supply has a similar waveform with
respect to the AC voltage; an on-period current detecting circuit
arranged to detect a current flowing through the switching element
or the inductor during an on period of the switching element or an
off-period current detecting circuit arranged to detect a current
flowing through the inductor during an off period of the switching
element; and an input current detecting circuit arranged to detect,
as an average value of the input current, a value that is obtained
by calculating ILav=Is+(Vi/L).times.Ton/2 or
ILav=Is+{(Vo-Vi)/L}.times.Toff/2, where a current value that is
obtained by sampling a current detection signal generated by the
on-period current detecting circuit or the off-period current
detecting circuit at a start point of the on period or an end point
of the off period of the switching element is represented by Is, an
input voltage input from the AC input power supply is represented
by Vi, an inductance value of the inductor is represented by L, the
on period of the switching element is represented by Ton, the off
period is represented by Toff, and an average value of a current
flowing through the inductor is represented by ILav.
2. The PFC converter according to claim 1, wherein the on-period
current detecting circuit includes a current detecting resistor
that is connected in series to the switching element.
3. The PFC converter according to claim 1, wherein the on-period
current detecting circuit includes a current transformer that is
connected in series to the switching element.
4. The PFC converter according to claim 1, wherein the off-period
current detecting circuit includes a current detecting resistor
that is connected in series to a current path in which a current
flows through the inductor during the off period of the switching
element.
5. The PFC converter according to claim 1, wherein the off-period
current detecting circuit includes a current transformer that is
connected in series to a current path in which a current flows
through the inductor during the off period of the switching
element.
6. A PFC converter comprising: a rectifier circuit arranged to
rectify an AC voltage input from an AC input power supply; a series
circuit connected in a subsequent stage of the rectifier circuit
and including an inductor and a switching element; a rectifying and
smoothing circuit connected in parallel to the switching element; a
switching control circuit arranged to perform on/off control on the
switching element so that an average value of an input current
input from the AC input power supply has a similar waveform with
respect to the AC voltage; an on-period current detecting circuit
arranged to detect a current flowing through the switching element
or the inductor during an on period of the switching element or an
off-period current detecting circuit arranged to detect a current
flowing through the inductor during an off period of the switching
element; and an input current detecting circuit arranged to detect,
as an average value of the input current, a value that is obtained
by calculating ILav=Is+(Vi/L).times.Ton/2 or
ILav=Is+{(Vo-Vi)/L}.times.Toff/2, where a current value that is
obtained by sampling a current detection signal generated by the
on-period current detecting circuit or the off-period current
detecting circuit at an end point of the on period or a start point
of the off period of the switching element is represented by Is, an
input voltage input from the AC input power supply is represented
by Vi, an output voltage is represented by Vo, an inductance value
of the inductor is represented by L, the on period of the switching
element is represented by Ton, the off period is represented by
Toff, and an average value of a current flowing through the
inductor is represented by ILav.
7. The PFC converter according to claim 6, wherein the on-period
current detecting circuit includes a current detecting resistor
that is connected in series to the switching element.
8. The PFC converter according to claim 6, wherein the on-period
current detecting circuit includes a current transformer that is
connected in series to the switching element.
9. The PFC converter according to claim 6, wherein the off-period
current detecting circuit includes a current detecting resistor
that is connected in series to a current path in which a current
flows through the inductor during the off period of the switching
element.
10. The PFC converter according to claim 6, wherein the off-period
current detecting circuit includes a current transformer that is
connected in series to a current path in which a current flows
through the inductor during the off period of the switching
element.
11. A PFC converter comprising: a first series circuit connected in
parallel to a load and including a first switching element and a
first rectifier element; an inductor connected between a connection
point of the first switching element and the first rectifier
element and a first input terminal of an AC input power supply; a
second series circuit connected in parallel to the load, including
a second switching element and a second rectifier element, and
including a connection point of the second switching element and
the second rectifier element that is connected to a second input
terminal of the AC input power supply; a smoothing circuit
connected in parallel to the load; a switching control circuit
arranged to perform on/off control on the first and second
switching elements so that an average value of an input current
input from the AC input power supply has a similar waveform with
respect to an AC voltage of the AC input power supply; an on-period
current detecting circuit arranged to detect a current flowing
through the first or second switching element or the inductor
during an on period of the first or second switching element or an
off-period current detecting circuit arranged to detect a current
flowing through the inductor during an off period of the first or
second switching element; and an input current detecting circuit
arranged to detect, as an average value of the input current, a
value that is obtained by calculating ILav=Is+(Vi/L).times.Ton/2 or
ILav=Is+{(Vo-Vi)/L}.times.Toff/2, where a current value that is
obtained by sampling a current detection signal generated by the
on-period current detecting circuit or the off-period current
detecting circuit at a start point of the on period or an end point
of the off period of the first or second switching element is
represented by Is, an input voltage input from the AC input power
supply is represented by Vi, an inductance value of the inductor is
represented by L, the on period of the first or second switching
element is represented by Ton, the off period is represented by
Toff, and an average value of a current flowing through the
inductor is represented by ILav.
12. The PFC converter according to claim 11, wherein the on-period
current detecting circuit includes a current detecting resistor
that is connected in series to the first and second switching
elements.
13. The PFC converter according to claim 11, wherein the on-period
current detecting circuit includes a current transformer that is
connected in series to the first and second switching elements.
14. The PFC converter according to claim 11, wherein the off-period
current detecting circuit includes a current detecting resistor
that is connected in series to a current path in which a current
flows through the inductor during the off period of the first and
second switching elements.
15. The PFC converter according to claim 11, wherein the off-period
current detecting circuit includes a current transformer that is
connected in series to a current path in which a current flows
through the inductor during the off period of the first and second
switching elements.
16. A PFC converter comprising: a first series circuit connected in
parallel to a load and including a first switching element and a
first rectifier element; an inductor connected between a connection
point of the first switching element and the first rectifier
element and a first input terminal of an AC input power supply; a
second series circuit connected in parallel to the load, including
a second switching element and a second rectifier element, and
including a connection point of the second switching element and
the second rectifier element that is connected to a second input
terminal of the AC input power supply; a smoothing circuit
connected in parallel to the load; a switching control circuit
arranged to perform on/off control on the first and second
switching elements so that an average value of an input current
input from the AC input power supply has a similar waveform with
respect to an AC voltage of the AC input power supply; an on-period
current detecting circuit arranged to detect a current flowing
through the first or second switching element or the inductor
during an on period of the first or second switching element or an
off-period current detecting circuit arranged to detect a current
flowing through the inductor during an off period of the first or
second switching element; and an input current detecting circuit
arranged to detect, as an average value of the input current, a
value that is obtained by calculating ILav=Is+(Vi/L).times.Ton/2 or
ILav=Is+{(Vo-Vi)/L}.times.Toff/2, where a current value that is
obtained by sampling a current detection signal generated by the
on-period current detecting circuit or the off-period current
detecting circuit at an end point of the on period or a start point
of the off period of the first or second switching element is
represented by Is, an input voltage input from the AC input power
supply is represented by Vi, an output voltage is represented by
Vo, an inductance value of the inductor is represented by L, the on
period of the first or second switching element is represented by
Ton, the off period is represented by Toff, and an average value of
a current flowing through the inductor is represented by ILav.
17. The PFC converter according to claim 16, wherein the on-period
current detecting circuit includes a current detecting resistor
that is connected in series to the first and second switching
elements.
18. The PFC converter according to claim 16, wherein the on-period
current detecting circuit includes a current transformer that is
connected in series to the first and second switching elements.
19. The PFC converter according to claim 16, wherein the off-period
current detecting circuit includes a current detecting resistor
that is connected in series to a current path in which a current
flows through the inductor during the off period of the first and
second switching elements.
20. The PFC converter according to claim 16, wherein the off-period
current detecting circuit includes a current transformer that is
connected in series to a current path in which a current flows
through the inductor during the off period of the first and second
switching elements.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an alternating current to direct
current (AC-DC) converter that receives an AC power supply and that
outputs a DC voltage, and particularly, to a power-factor
correction (PFC) converter for correcting a power factor.
2. Description of the Related Art
In Japan and Europe, for example, harmonic current control that is
classified in accordance with an application or input power is
performed. In order to respond to such control, a circuit called a
PFC converter is added to a power supply of general home appliances
subjected to harmonic current control, whereby measures have been
taken to suppress harmonic currents.
In a typical switching power supply device using a commercial AC
power supply as an input power supply, the commercial AC power
supply is rectified and smoothed so as to be converted to a DC
voltage, and switching by a DC-DC converter is performed on the DC
voltage. Therefore, an input current is discontinuous and is
significantly distorted as compared to a sinusoidal wave. This
causes a harmonic current.
For the purpose of suppressing the harmonic current, a PFC
converter is provided in a stage after a full-wave rectifier
circuit and before a smoothing circuit that includes a smoothing
capacitor.
This PFC converter, which includes a chopper circuit, operates so
that an input current waveform is similar to an input voltage
waveform, i.e., so that the waveforms are sinusoidal waveforms
having the same phase. Accordingly, a harmonic current is
suppressed to a certain level or lower.
An example configuration of the PFC converter disclosed in Japanese
Unexamined Patent Application Publication No. 2004-282958 will be
described with reference to FIG. 1.
In the power-factor correction circuit illustrated in FIG. 1, a
series circuit including a step-up reactor L1, a switching element
Q1 defined by a MOSFET, and a current detecting resistor R is
connected to both output terminals of a diode bridge B1 that
rectifies an AC power supply voltage of an AC input power supply
Vac1. A series circuit including a diode D1 and a smoothing
capacitor C1 is connected to both ends of the switching element Q1,
and a load RL is connected to both ends of the smoothing capacitor
C1. The switching element Q1 is turned on/off under pulse width
modulation (PWM) control by a control circuit 10. The current
detecting resistor R detects an input current flowing through the
diode bridge B1.
The control circuit 10 includes an error amplifier 111, a
multiplier 112, an error amplifier 113, a voltage controlled
oscillator (VCO) 115, and a PWM comparator 116.
The error amplifier 111 calculates an error between the voltage of
the smoothing capacitor C1 and a reference voltage E1. The
multiplier 112 multiplies an error voltage signal by a voltage
rectified by the diode bridge B1. The error amplifier 113 generates
an error between a multiplication result generated by the
multiplier 112 and a current signal flowing through the diode
bridge B1 and outputs the error to the PWM comparator 116.
The VCO 115 generates a triangular-wave signal of a frequency
according to a voltage value of a rectified AC power supply
voltage.
In the PWM comparator 116, a triangular-wave signal from the VCO
115 is input to a minus terminal, and a signal from the error
amplifier 113 is input to a plus terminal. That is, the PWM
comparator 116 applies a duty pulse according to a current flowing
through the diode bridge B1 and an output voltage to the switching
element Q1. This duty pulse is a pulse-width control signal that
continuously compensates for fluctuations of an AC power supply
voltage and a DC load voltage in constant cycles. With this
configuration, control is performed so that the waveform of an AC
power supply current matches the waveform of an AC power supply
voltage, whereby the power factor is corrected.
On the other hand, a digitally-controlled PFC converter is
disclosed in Japanese Unexamined Patent Application Publication No.
7-177746.
In the case of digital control, a current flowing through an
inductor is detected, and switching is performed on a switching
element under PWM control according to the current value.
As described above, in the PFC converter, it is necessary to detect
a current flowing through an inductor (hereinafter "inductor
current") in order to cause an input current waveform to be similar
to an input voltage waveform. For this purpose, the following
methods may be typically used.
(a) Directly detect a current flowing through an inductor.
(b) Detect a current flowing through a switching element and treat
the current as being equivalent to an inductor current.
(c) Detect a current flowing through a diode provided on an output
side and treat the current as being equivalent to an inductor
current.
In addition, the following methods may be used to provide a current
detecting means.
(1) Insert a current detecting resistor in series into a current
path and detect a decreased voltage at both ends of the
resistor.
(2) Insert a current transformer into a current path or perform
detection using a current transformer in which an inductor is on
the primary side.
(3) Provide a hole sensor in a current path and detect an output
voltage thereof.
In the above-described method (1) of using a current detecting
resistor, power consumption in the current detecting resistor
itself is loss, which is a problem in terms of reducing loss. In
the method (2) of using a current transformer, a DC component of a
current that is to be detected is cut, and thus, only an AC
component of the current is detected and the DC component of the
current (DC offset) cannot be detected. If each of the currents in
the above-described methods (b) and (c) is detected using a current
transformer and signals are combined, an inductor current can be
detected. However, in this case, two current transformers are
required. In the method (3) of using a hole sensor, the problems
that may occur in the methods (1) and (2) do not arise, but the
overall cost increases because the sensor is expensive.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of
the present invention provide a low-cost PFC converter that
accurately detects an inductor current including a DC component and
appropriately corrects a power factor with low loss.
A PFC converter according to a preferred embodiment of the present
invention preferably includes a rectifier circuit arranged to
rectify an AC voltage input from an AC input power supply, a series
circuit that is connected in a subsequent stage of the rectifier
circuit and that includes an inductor and a switching element, a
rectifying and smoothing circuit connected in parallel to the
switching element, a switching control circuit arranged to perform
on/off control of the switching element so that an average value of
an input current input from the AC input power supply has a similar
waveform with respect to the AC voltage, and an on-period current
detecting circuit arranged to detect a current flowing through the
switching element or the inductor during an on period of the
switching element or an off-period current detecting circuit
arranged to detect a current flowing through the inductor during an
off period of the switching element. The PFC converter preferably
has any of the following configurations.
The PFC converter preferably includes an input current detecting
circuit arranged to detect, as an average value of the input
current, a value that is obtained by sampling a current detection
signal generated by the on-period current detecting circuit or the
off-period current detecting circuit at the approximate middle of
the on period or off period of the switching element.
The value obtained by sampling the detection signal of the current
flowing through the switching element at the approximate middle of
the on period of the switching element is an average value of an
inductor current. Also, the value obtained by sampling the
detection signal of the current flowing through the inductor at the
approximate middle of the off period of the switching element is an
average value of an inductor current. In any case, the average
value of the current input to the PFC converter can preferably be
detected substantially by using one-point sampling.
In a case in which a current value that is obtained by sampling a
current detection signal generated by the on-period current
detecting circuit or the off-period current detecting circuit at a
start point of the on period or an end point of the off period of
the switching element is represented by Is, an input voltage input
from the AC input power supply is represented by Vi, an inductance
value of the inductor is represented by L, the on period of the
switching element is represented by Ton, the off period is
represented by Toff, and an average value of a current flowing
through the inductor is represented by ILav, a value that is
preferably obtained by calculating ILav=Is+(Vi/L).times.Ton/2 or
ILav=Is+{(Vo-Vi)/L}.times.Toff/2 is detected as an average value of
the input current.
Accordingly, the average value of the current input to the PFC
converter is substantially detected.
In a case in which a current value that is obtained by sampling a
current detection signal generated by the on-period current
detecting circuit or the off-period current detecting circuit at an
end point of the on period or a start point of the off period of
the switching element is represented by Is, an input voltage input
from the AC input power supply is represented by Vi, an output
voltage is represented by Vo, an inductance value of the inductor
is represented by L, the on period of the switching element is
represented by Ton, the off period is represented by Toff, and an
average value of a current flowing through the inductor is
represented by ILav, a value that is preferably obtained by
calculating ILav=Is-(Vi/L).times.Ton/2 or
ILav=Is-{(Vo-Vi)/L}.times.Toff/2 is detected as an average value of
the input current.
Accordingly, the average value of the current input to the PFC
converter can be substantially detected.
A PFC converter according to another preferred embodiment of the
present invention preferably includes a first series circuit that
is connected in parallel to a load and that includes a first
switching element and a first rectifier element, an inductor that
is connected between a connection point of the first switching
element and the first rectifier element and a first input terminal
of an AC input power supply, a second series circuit that is
connected in parallel to the load, that includes a second switching
element and a second rectifier element, and that includes a
connection point of the second switching element and the second
rectifier element, the connection point being connected to a second
input terminal of the AC input power supply, a smoothing circuit
connected in parallel to the load, a switching control circuit
arranged to perform on/off control on the first and second
switching elements so that an average value of an input current
input from the AC input power supply has a similar waveform with
respect to an AC voltage of the AC input power supply, and an
on-period current detecting circuit arranged to detect a current
flowing through the switching element or the inductor during an on
period of the switching element or an off-period current detecting
circuit arranged to detect a current flowing through the inductor
during an off period of the switching element. The PFC converter of
the second type preferably has any of the following
configurations.
The PFC converter preferably includes an input current detecting
circuit arranged to detect, as an average value of the input
current, a value that is obtained by sampling a current detection
signal generated by the on-period current detecting circuit or the
off-period current detecting circuit at the approximate middle of
the on period or off period of the first or second switching
element.
The value obtained by sampling the detection signal of the current
flowing through the first or second switching element at the
approximate middle of the on period or off period of the first or
second switching element is an average value of an inductor
current. Thus, the average value of the current flowing through the
inductor can preferably be detected substantially by using
one-point sampling.
The PFC converter preferably includes an input current detecting
circuit arranged to detect, as an average value of the input
current, a value that is preferably obtained by calculating
ILav=Is+(Vi/L).times.Ton/2 or ILav=Is+{(Vo-Vi)/L}.times.Toff/2, in
a case in which a current value that is obtained by sampling a
current detection signal generated by the on-period current
detecting circuit or the off-period current detecting circuit at a
start point of the on period or an end point of the off period of
the switching element is represented by Is, an input voltage input
from the AC input power supply is represented by Vi, an inductance
value of the inductor is represented by L, the on period of the
switching element is represented by Ton, the off period is
represented by Toff, and an average value of a current flowing
through the inductor is represented by ILav.
Accordingly, the average value of the current input to the PFC
converter can preferably be substantially detected.
The PFC converter preferably includes an input current detecting
circuit arranged to detect, as an average value of the input
current, a value that is preferably obtained by calculating
ILav=Is-(Vi/L).times.Ton/2 or ILav=Is-{(Vo-Vi)/L}.times.Toff/2, in
a case in which a current value that is obtained by sampling a
current detection signal generated by the on-period current
detecting circuit or the off-period current detecting circuit at an
end point of the on period or a start point of the off period of
the switching element is represented by Is, an input voltage input
from the AC input power supply is represented by Vi, an output
voltage is represented by Vo, an inductance value of the inductor
is represented by L, the on period of the switching element is
represented by Ton, the off period is represented by Toff, and an
average value of a current flowing through the inductor is
represented by ILav.
Accordingly, the average value of the current input to the PFC
converter can preferably be substantially detected.
The on-period current detecting circuit preferably includes a
current detecting resistor that is connected in series to the
switching element.
With this configuration, the average value of the current flowing
through the switching element during the on period of the switching
element (i.e., inductor current) can be obtained.
The on-period current detecting circuit preferably includes a
current transformer that is connected in series to the first or
second switching element, for example.
With this configuration, the average value of the current flowing
through the inductor during the on period of the switching element
can be obtained.
The off-period current detecting circuit preferably includes a
current detecting resistor that is connected in series to a current
path in which a current flows through the inductor during the off
period of the switching element, for example.
With this configuration, the average value of the current flowing
through the inductor during the off period of the switching element
can be obtained.
The off-period current detecting circuit preferably includes a
current transformer that is connected in series to a current path
in which a current flows through the inductor during the off period
of the switching element, for example.
With this configuration, the average value of the current flowing
through the inductor during the off period of the switching element
can be obtained.
In the PFC converter according to various preferred embodiments of
the present invention, the cost is significantly reduced by using a
current detecting resistor. In addition, since the current
detecting resistor is used, an inductor current including a DC
component can be detected, and a power factor can be appropriately
corrected. Further, a current flows through the current detecting
resistor only during any of the on period and off period of the
switching element, and thus, the power loss caused by the current
detecting resistor is reduced. Furthermore, when using a current
transformer for current detection, only one current transformer is
required and, thus, the cost and size are reduced. Furthermore,
lower loss and simplification of the current detecting circuit are
achieved.
The above and other features, elements, characteristics and
advantages of the present invention will become more apparent from
the following detailed description of preferred embodiments of the
present invention with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of a known PFC converter described.
FIG. 2 is a circuit diagram of a PFC converter according to a first
preferred embodiment of the present invention.
FIGS. 3A to 3C include diagrams illustrating various methods for
controlling a PFC converter by a digital signal processing
circuit.
FIGS. 4A to 4C includes waveform diagrams of a voltage and current
of the PFC converter 101 in the unit of a switching cycle in a
state in which control is performed in a current continuous
mode.
FIG. 5 is a diagram illustrating a method for obtaining an average
value of a current flowing through an inductor L1 in order to
perform average current control in the current continuous mode.
FIG. 6 is a circuit diagram of a switching power supply device
according to a second preferred embodiment of the present
invention.
FIG. 7 is a diagram illustrating a method for obtaining an average
value of a current flowing through an inductor in order to perform
average current control in the current continuous mode.
FIG. 8 is a circuit diagram of a PFC converter according to a third
preferred embodiment of the present invention.
FIG. 9 is a waveform diagram of individual portions of the PFC
converter.
FIG. 10 is a circuit diagram of a PFC converter according to a
fourth preferred embodiment of the present invention.
FIGS. 11A to 11D include diagrams illustrating current paths of the
PFC converter at four timings.
FIG. 12 is a diagram illustrating a method for obtaining an average
value of a current flowing through a switching element in a PFC
converter according to a fifth preferred embodiment of the present
invention.
FIG. 13 is a diagram illustrating a method for obtaining an average
value of a current flowing through an inductor in a PFC converter
according to a sixth preferred embodiment of the present
invention.
FIG. 14 is a diagram illustrating a method for obtaining an average
value of a current flowing through a switching element in a PFC
converter according to a seventh preferred embodiment of the
present invention.
FIG. 15 is a diagram illustrating a method for obtaining an average
value of a current flowing through an inductor in a PFC converter
according to an eighth preferred embodiment of the preset
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Preferred Embodiment
A PFC converter according to a first preferred embodiment of the
present invention will be described with reference to FIGS. 2 to
4.
FIG. 2 is a circuit diagram of the PFC converter according to the
first preferred embodiment. In FIG. 2, symbols P11 and P12 denote
input ports of the PFC converter 101, and symbols P21 and P22
denote output ports of the PFC converter 101. An AC input power
supply Vac, which is a commercial AC power supply, is input to the
input ports P11 and P12, and a load circuit 20 is connected to the
output ports P21 and P22.
The load circuit 20 is, for example, a circuit of a DC-DC converter
and an electronic apparatus that is supplied with power by the
DC-DC converter.
In an input stage of the PFC converter 101, a diode bridge B1 that
performs full-wave rectification on an AC voltage of the AC input
power supply Vac is preferably provided. On the output side of the
diode bridge B1, a series circuit including an inductor L1, a
switching element Q1, and a current detecting resistor R1 is
connected. A rectifying and smoothing circuit including a diode D1
and a smoothing capacitor C1 is connected in parallel to both ends
of the switching element Q1. The inductor L1, the switching element
Q1, the diode D1, and the smoothing capacitor C1 define a step-up
chopper circuit.
An input voltage detecting circuit 11 is provided between both ends
on the output side of the diode bridge B1. Also, an output voltage
detecting circuit 12 is provided between the output ports P21 and
P22. A digital signal processing circuit 13 is preferably defined
by a digital signal processor (DSP) and controls the PFC converter
101 through digital signal processing. That is, the digital signal
processing circuit 13 receives an output signal of the input
voltage detecting circuit 11 and detects the phase of the voltage
of the AC input power supply using the method described below. In
addition, the digital signal processing circuit 13 receives an
output signal of the output voltage detecting circuit 12 and
detects an output voltage. Further, the digital signal processing
circuit 13 turns on/off the switching element Q1 at a predetermined
switching frequency.
Furthermore, the digital signal processing circuit 13 preferably
includes a port arranged to communicate with the load circuit 20
and to perform data communication or input/output of signals,
thereby constantly transmitting a status of the converter to the
load circuit (electronic apparatus), transmitting an input voltage,
an output voltage, and an output current, for example, and
receiving a load status from the load circuit side to reflect it in
switching control.
FIGS. 3A to 3C include diagrams illustrating various methods for
controlling the PFC converter 101 used by the digital signal
processing circuit 13. Each of FIGS. 3A to 3C illustrates a current
waveform in one cycle of the AC input power supply. Here, a
waveform IL is the waveform of a current flowing through the
inductor L1 in the PFC converter 101 illustrated in FIG. 2. Ip
denotes an envelope of the peak values (peak current) thereof, and
Ia denotes an envelope of the average values (average current).
Note that, for convenience of illustration, FIGS. 3A to 3C
illustrate a case in which the switching frequency of the PFC
converter 101 is extremely low, that is, a frequency at which the
waveform of a current flowing through the inductor L1 is shown as a
triangular waveform.
FIG. 3A is a waveform diagram in a current continuous mode, FIG. 3B
is a waveform diagram in a current discontinuous mode, and FIG. 3C
is a waveform diagram in a current critical mode. As can be seen,
in the current continuous mode illustrated in FIG. 3A, the current
flowing through the inductor L1 of the PFC converter 101 is not
zero except at the vicinity of a zero crossing point of the AC
input power supply. In the current discontinuous mode illustrated
in FIG. 3B a period in which the current value becomes zero occurs
every time excitation energy is accumulated in/emitted from the
inductor L1 of the PFC converter 101. In the critical mode
illustrated in FIG. 3C, the current value becomes zero every time
excitation energy is accumulated in/emitted from the inductor L1,
and a state in which the current value is zero does not
continue.
In the first preferred embodiment, the digital signal processing
circuit 13 preferably performs control in the current continuous
mode illustrated in FIG. 3A or in the current critical mode
illustrated in FIG. 3C.
FIGS. 4A to 4C include waveform diagrams of a voltage and current
of the PFC converter 101 in the unit of a switching cycle in a
state in which control is performed in the current continuous
mode.
The digital signal processing circuit 13 preferably performs
switching control so that a current input to the PFC converter 101,
that is, an average value of the current flowing through the
inductor L1, has a waveform similar to a full-wave rectification
waveform. In this manner, an input current of which the waveform is
similar to that of an input voltage flows, so that harmonics are
prevented and the power factor is corrected.
FIG. 4A is a current waveform of an average value Ii of a current
flowing through the inductor L1 in the unit of a half cycle of a
commercial power supply frequency, FIG. 4B is an enlargement of a
portion of the time axis in the waveform in FIG. 4A, that is, a
waveform diagram of a current IL flowing through the inductor L1 in
the unit of a switching cycle, and FIG. 4C is a waveform diagram of
a drain-source voltage Vds of the switching element Q1.
During an on period Ton of the switching element Q1, the current IL
flows through the inductor L1, and the current IL increases with a
slope determined in accordance with a voltage between both ends of
the inductor L1 and the inductance of the inductor L1. After that,
during an off period Toff of the switching element Q1, the current
IL decreases with a slope determined in accordance with a voltage
between both ends of the inductor L1 and the inductance thereof. In
this manner, the current IL flowing through the inductor L1
fluctuates in the switching cycle within the width of a current
ripple .DELTA.IL.
FIG. 5 is a diagram illustrating a method for obtaining an average
value of the current flowing through the inductor L1 in order to
perform average current control in the current continuous mode.
The value of a current flowing through the inductor L1 at a
turn-off timing of the switching element Q1 (peak value) is
represented by ILp, and the value of a current flowing through the
inductor L1 at a turn-on timing of the switching element Q1
(minimum value) is represented by ILb. In this case, an average
value of the current (average current) flowing through the inductor
L1 during the off period Toff of the switching element Q1 is
preferably represented by the following relationship.
ILav=(ILp+ILb)/2 (1)
On the other hand, the current flowing through the inductor L1
linearly decreases during the off period Toff of the switching
element Q1, and thus, the current value of the inductor L1 at an
approximate middle timing of the off period Toff of the switching
element Q1 is substantially equal to the foregoing average current
ILav.
Then, a decreased voltage of the resistor R1 at the approximate
middle timing of the off period Toff of the switching element Q1 is
subjected to sampling. This sampling value is a value that is
proportional to the average current ILav of the current flowing
through the inductor L1 during the off period Toff of the switching
element Q1. A switching control signal that is provided to the gate
of the switching element Q1 is generated by the digital signal
processing circuit 13, and thus the approximate middle timing of
the off period Toff is recognized and managed by the digital signal
processing circuit 13. Therefore, a decreased voltage of the
current detecting resistor R1 can be subjected to sampling at the
approximate middle timing of the off period Toff without inputting
a timing signal from the outside, for example.
Second Preferred Embodiment
FIG. 6 is a circuit diagram of a switching power supply device 201
according to a second preferred embodiment of the present
invention.
In FIG. 6, the switching power supply device 201 preferably
includes a PFC converter 102 and a DC-DC converter 50. A load 60 is
connected to the output of the DC-DC-converter 50.
An AC input power supply Vac, which is a commercial AC power
supply, is input to input ports P11 and P12 of the PFC converter
102, and the DC-DC converter 50 is connected to the output portion.
The PFC converter 102 preferably includes a diode bridge B1
arranged to perform full-wave rectification on the AC input power
supply Vac, an inductor L1 connected to the output of the diode
bridge B1, a switching element Q1, a diode D1, and a smoothing
capacitor C1. The inductor L1, the switching element Q1, the diode
D1, and the smoothing capacitor C1 define a step-up chopper
circuit.
In contrast to the first preferred embodiment illustrated in FIG.
2, in the second preferred embodiment, a current detecting resistor
R2 is preferably connected in series to the source side of the
switching element Q1.
The DC-DC converter 50 preferably includes a rectifying and
smoothing circuit including a transformer T1, a switching element
Q2 connected in series to a primary coil Lp of the transformer T1,
and a diode D2 and capacitor C2 connected to a secondary coil Ls of
the transformer T1.
In addition, a digital signal processing circuit provided on the
primary side of the transformer T1 preferably not only outputs a
switching control signal to the switching element Q1 of the PFC
converter 102, but also outputs a switching control signal to the
switching element Q2 of the DC-DC converter 50. Also, the digital
signal processing circuit 13 receives a detection signal from an
output voltage detecting circuit 12 via an insulation circuit 16,
thereby detecting an output voltage. Then, the digital signal
processing circuit 13 controls the on-duty ratio of the switching
element Q2, thereby stabilizing an output voltage of the DC-DC
converter 50.
Furthermore, the digital signal processing circuit receives a
detection voltage of an input voltage detecting circuit 11 of the
PFC converter 102, a detection signal of the output voltage
detecting circuit 12, and a decreased voltage of the current
detecting resistor R2, and accordingly, supplies a switching
control signal for an on period and an off period to the gate of
the switching element Q1. That is, the digital signal processing
circuit 13 controls the switching element Q1 so that a current
input to the PFC converter 102 has a waveform similar to that of an
input voltage. Also, the digital signal processing circuit 13
communicates with the load 60 via an insulation circuit 17, thereby
transmitting a status of the PFC converter 102 and the DC-DC
converter 50 to the load 60 or receiving a load status from the
load 60 to reflect it in switching control.
FIG. 7 is a diagram illustrating a method for obtaining an average
value of a current flowing through the inductor L1 in order to
perform average current control in the current continuous mode.
A drain current flowing through the switching element Q1 at a
turn-on timing of the switching element Q1 is represented by IDb,
and the value of a current flowing through the switching element Q1
just before a turn-off timing of the switching element Q1 (peak
value) is represented by IDp. In this case, an average value of the
current flowing through the switching element Q1 during an on
period Ton of the switching element Q1, that is, the current
flowing through the inductor L1 (average current), is preferably
represented by the following relationship. ILav=(IDp+IDb)/2 (2)
On the other hand, the current flowing through the inductor L1
linearly increases during the on period Ton of the switching
element Q1, and thus, the current value of the switching element Q1
at the approximate middle timing of the on period Ton of the
switching element Q1 is substantially equal to the average current
ILav.
Then, a decreased voltage of the current detecting resistor R2 at
the approximate middle timing of the on period Ton of the switching
element Q1 is preferably subjected to sampling. This sampling value
is a value that is proportional to the average current ILav of the
current flowing through the inductor L1 during the on period Ton of
the switching element Q1. A switching control signal that is
provided to the gate of the switching element Q1 is generated by
the digital signal processing circuit 13, and thus, the approximate
middle timing of the on period Ton is preferably recognized and
managed by the digital signal processing circuit 13. Therefore, a
decreased voltage of the resistor R2 is subjected to sampling at
the approximate middle timing of the on period Ton without
inputting a timing signal from the outside, for example.
Third Preferred Embodiment
FIG. 8 is a circuit diagram of a PFC converter 103 according to a
third preferred embodiment of the present invention. FIG. 9 is a
waveform diagram of individual portions of the PFC converter
103.
The PFC converter 103 preferably includes three inductors L11, L12,
and L13, three diodes D11, D12, and D13, and three switching
elements Q11, Q12 and Q13, thereby defining a three-phase PFC
converter. A digital signal processing circuit 13 detects an input
voltage using an input voltage detecting circuit 11, receives
decreased voltages from current detecting resistors R11, R12, and
R13, and supplies switching control signals to the switching
elements Q11, Q12, and Q13, respectively.
FIG. 9 is a waveform diagram of inductor currents IL11, IL12, and
IL13 flowing through the three inductors L11, L12, and L13 of the
PFC converter 103 illustrated in FIG. 8, and an input current IL.
By sequentially turning on/off the three switching elements Q11,
Q12, and Q13 with a phase difference of 120 degrees, electrical
stresses of an input capacitor C1 and an output capacitor C0 are
alleviated and noise is reduced. In addition, energy (heat) is
dispersed, so that the size and height of components can be
decreased.
In order to disperse the energy (heat), it is preferable that the
inductor currents IL11, IL12, and IL13 flowing through the three
inductors L11, L12, and L13 are even, and it is necessary to
accurately detect the individual inductor currents. In the third
preferred embodiment, as illustrated in FIG. 9, the digital signal
processing circuit 13 preferably performs sampling at the
approximate middle timing of an on period at which currents flow
through the three switching elements Q11, Q12, and Q13, thereby
accurately detecting the average values of the inductor currents of
the three inductors.
Specifically, the digital signal processing circuit 13 samples the
decreased voltage of the current detecting resistor R11 at an
approximate middle timing ts1 of the on period of the switching
element Q11, thereby obtaining the average value of the current
IL11 flowing through the inductor L11. Also, the digital signal
processing circuit 13 samples the decreased voltage of the current
detecting resistor R12 at an approximate middle timing ts2 of the
on period of the switching element Q12, thereby obtaining the
average value of the current IL12 flowing through the inductor L12.
Also, the digital signal processing circuit 13 samples the
decreased voltage of the current detecting resistor R13 at an
approximate middle timing ts3 of the on period of the switching
element Q13, thereby obtaining the average value of the current
IL13 flowing through the inductor L13.
Fourth Preferred Embodiment
FIG. 10 is a circuit diagram of a PFC converter 104 according to a
fourth preferred embodiment. FIG. 11 includes diagrams illustrating
current paths of the PFC converter 104 at four timings.
The PFC converter 104 illustrated in FIG. 10 preferably is a
diode-bridgeless PFC converter in which an inductor and two
switching elements are provided without via a diode bridge, for
example.
In FIG. 10, symbols P11 and P12 denote input ports of the PFC
converter 104, and symbols P21 and P22 denote output ports of the
PFC converter 104. An AC input power supply Vac, which is a
commercial AC power supply, is input to the input ports P11 and
P12, and a load circuit 20 is connected to the output ports P21 and
P22.
The load circuit 20 includes, for example, a circuit of a DC-DC
converter and an electronic apparatus that is supplied with power
by the DC-DC converter.
In an input stage of the PFC converter 104, preferably, an input
voltage detecting circuit 11 is provided, and an inductor L1 is
connected in series to one of the lines thereof. In the subsequent
stage of the inductor L1, a bridge circuit including diodes D1 and
D2 and switching elements Q1 and Q2 is connected. Current detecting
resistors R21 and R22 are connected between the sources of the
switching elements Q1 and Q2 and a ground. A smoothing circuit
including a smoothing capacitor C1 is preferably connected in
parallel to the output of the bridge circuit.
FIG. 11A illustrates a current path when both of the switching
elements Q1 and Q2 are in an on state in a positive half cycle of
the AC input power supply, and FIG. 11B illustrates a current path
when both of the switching elements Q1 and Q2 are in an off state
in a positive half cycle of the AC input power supply.
FIG. 11C illustrates a current path when both of the switching
elements Q1 and Q2 are in an on state in a negative half cycle of
the AC input power supply, and FIG. 11D illustrates a current path
when both of the switching elements Q1 and Q2 are in an off state
in a negative half cycle of the AC input power supply.
In a positive half cycle of the AC input power supply, a current
flows through the path illustrated in FIG. 11A and excitation
energy is accumulated in the inductor L1 when the switching
elements Q1 and Q2 are in an on state, whereas a current flows
through the path illustrated in FIG. 11B and excitation energy is
emitted from the inductor L1 when the switching elements Q1 and Q2
are in an off state. At this time, a current flows via a parasitic
diode of the switching element Q2. Likewise, in a negative half
cycle of the AC input power supply, a current flows through the
path illustrated in FIG. 11C and excitation energy is accumulated
in the inductor L1 when the switching elements Q1 and Q2 are in an
on state, whereas excitation energy is emitted from the inductor L1
at the timing illustrated in FIG. 11D when the switching elements
Q1 and Q2 are in an off state. At this time, a current flows via a
parasitic diode of the switching element Q1.
Preferably, the current detecting resistor R21 is provided to
detect the current flowing through the inductor L1 during an on
period of the switching element Q1 in a positive half cycle of the
AC input power supply. Also, preferably, the current detecting
resistor R22 is provided to detect the current flowing through the
inductor L1 during an on period of the switching element Q2 in a
negative half cycle of the AC input power supply. The digital
signal processing circuit 13 illustrated in FIG. 10 samples the
decreased voltage of the current detecting resistors R21 and R22 at
the approximate middle of the on period of the switching elements
Q1 and Q2, thereby detecting the average value of the current
flowing through the inductor L1.
Also, preferably the current detecting resistor R1 is provided to
detect the current flowing through the inductor L1 during an off
period of the switching elements Q1 and Q2. The digital signal
processing circuit 13 samples the decreased voltage of the current
detecting resistor R1 at the approximate middle of the off period
of the switching elements Q1 and Q2, thereby detecting the average
value of the current flowing through the inductor L1.
In the positive half cycle of the AC input power supply, the
average value of the current flowing through the inductor L1 can be
detected by sampling the decreased voltage of the current detecting
resistor R22 at the approximate middle of the off period of the
switching elements Q1 and Q2. Likewise, in the negative half cycle
of the AC input power supply, the average value of the current
flowing through the inductor L1 can be detected by sampling the
decreased voltage of the current detecting resistor R21 at the
approximate middle of the off period of the switching elements Q1
and Q2. Therefore, in this case, the current detecting resistor R1
is not required, and the power loss in the current detecting
resistor R1 is reduced.
In the example illustrated in FIGS. 10 to 11D, the inductor L1 is
preferably connected between a connection point of the diode D1 and
the switching element Q1 and the input port P11 of the AC input
power supply. Alternatively, an inductor may also be connected
between a connection point of the diode D2 and the switching
element Q2 and the input port P12 of the AC input power supply.
Fifth Preferred Embodiment
In a fifth preferred embodiment of the present invention, a current
flowing through a switching element is preferably detected at the
start point of an on period or at the end point of an off period of
the switching element, thereby detecting an average value of a
current input to a PFC converter.
The circuit configuration can be applied to the configuration of
the PFC converter 102 illustrated in FIG. 6 according to the second
preferred embodiment. Here, a description will be given of a
process executed by the digital signal processing circuit 13 in the
PFC converter 102 illustrated in FIG. 6 with reference to FIG.
6.
FIG. 12 is a diagram illustrating a method for obtaining an average
value of a current input to the PFC converter 102 in order to
perform average current control in the current continuous mode.
The value of a drain current flowing through the switching element
Q1 at the start point of an on period of the switching element Q1
is represented by Is, an input voltage input from the AC input
power supply is represented by Vi, the inductance value of the
inductor L1 is represented by L, the on period of the switching
element Q1 is represented by Ton, and an average value of the
current flowing through the inductor L1 is represented by ILav.
In this case, the slope of change in the current flowing through
the inductor L1 during the on period of the switching element Q1 is
represented by Vi/L. Thus, the average value of the current input
to the PFC converter 102 is preferably obtained by calculating
ILav=Is+(Vi/L).times.Ton/2, by sampling the current Is flowing
through the switching element or inductor just after the turn-on of
the switching element Q1.
The inductance value L is known at the time of shipment. Ton is
recognized and managed by the digital signal processing circuit 13,
as described above. Vi is originally detected to form a sinusoidal
wave of a current. Normally, division imposes a large processing
load on the digital signal processing circuit. However, for
division based on the exponentiation of two, processing can be
performed at high speed as in a product-sum operation by using bit
shift. For the above-described reasons, this method enables
execution of a calculation without adding a circuit or increasing a
processing load.
Sixth Preferred Embodiment
In a sixth preferred embodiment of the present invention, a current
flowing through an inductor is preferably detected at the start
point of an on period or at the end point of an off period of a
switching element, thereby detecting an average value of a current
input to a PFC converter.
The circuit configuration can be applied to the configuration of
the PFC converter 101 illustrated in FIG. 2 according to the first
preferred embodiment. Here, a description will be given of a
process executed by the digital signal processing circuit 13 in the
PFC converter 101 illustrated in FIG. 2 with reference to FIG.
2.
FIG. 13 is a diagram illustrating a method for obtaining an average
value of a current input to the PFC converter 101 in order to
perform average current control in the current continuous mode.
The value of an inductor current flowing through the inductor L1 at
the end point of an off period of the switching element Q1 is
represented by Is, an input voltage input from the AC input power
supply is represented by Vi, the inductance value of the inductor
L1 is represented by L, the off period of the switching element Q1
is represented by Toff, and an average value of the current flowing
through the inductor L1 is represented by ILav.
In this case, the slope of change in the current flowing through
the inductor L1 during the off period of the switching element Q1
is represented by -(Vo-Vi)/L.
Thus, the average value of the current input to the PFC converter
101 is preferably obtained by calculating
ILav=Is+{(Vo-Vi)/L}.times.Toff/2, by sampling the current Is
flowing through the inductor just before the turn-on of the
switching element Q1. Seventh Preferred Embodiment
In a seventh preferred embodiment of the present invention, a
current flowing through a switching element is detected at the end
point of an on period or at the start point of an off period of the
switching element, thereby detecting an average value of a current
input to a PFC converter.
The circuit configuration can be applied to the configuration of
the PFC converter 102 illustrated in FIG. 6 according to the second
preferred embodiment. Here, a description will be given of a
process executed by the digital signal processing circuit 13 in the
PFC converter 102 illustrated in FIG. 6 with reference to FIG.
6.
FIG. 14 is a diagram illustrating a method for obtaining an average
value of a current input to the PFC converter 102 in order to
perform average current control in the current continuous mode.
The value of a drain current flowing through the switching element
Q1 at the end point of an on period of the switching element Q1 is
represented by Is, an input voltage input from the AC input power
supply is represented by Vi, the inductance value of the inductor
L1 is represented by L, the on period of the switching element Q1
is represented by Ton, and an average value of the current flowing
through the inductor L1 is represented by ILav.
In this case, the slope of change in the current flowing through
the inductor L1 during the on period of the switching element Q1 is
represented by Vi/L. Thus, the average value of the current input
to the PFC converter 102 is preferably obtained by calculating
ILav=Is-(Vi/L).times.Ton/2, by sampling the current Is flowing
through the switching element or inductor just before the turn-off
of the switching element Q1. Eighth Preferred Embodiment
In an eighth preferred embodiment of the present invention, a
current flowing through an inductor is detected at the end point of
an on period or at the start point of an off period of a switching
element, thereby detecting an average value of a current input to a
PFC converter.
The circuit configuration can be applied to the configuration of
the PFC converter 101 illustrated in FIG. 2 according to the first
preferred embodiment. Here, a description will be given of a
process executed by the digital signal processing circuit 13 in the
PFC converter 101 illustrated in FIG. 2 with reference to FIG.
2.
FIG. 15 is a diagram illustrating a method for obtaining an average
value of a current input to the PFC converter 101 in order to
perform average current control in the current continuous mode.
The value of an inductor current flowing through the inductor L1 at
the end point of an on period of the switching element Q1 is
represented by Is, an input voltage input from the AC input power
supply is represented by Vi, the inductance value of the inductor
L1 is represented by L, the off period of the switching element Q1
is represented by Toff, and an average value of the current flowing
through the inductor L1 is represented by ILav.
In this case, the slope of change in the current flowing through
the inductor L1 during the off period of the switching element Q1
is represented by -(Vo-Vi)/L.
Thus, the average value of the current input to the PFC converter
101 is preferably obtained by calculating
ILav=Is-{(Vo-Vi)/L}.times.Toff/2, by sampling the current Is
flowing through the inductor at the turn-off of the switching
element Q1.
In each of the above-described preferred embodiments, a current
flowing through a switching element or a current flowing through an
inductor is detected using a decreased voltage of a current
detecting resistor. However, when using a current transformer, the
primary side of the current transformer is connected to a current
path, and an output voltage on the secondary side of the current
transformer is used as a current detection signal.
While preferred embodiments of the present invention have been
described above, it is to be understood that variations and
modifications will be apparent to those skilled in the art without
departing from the scope and spirit of the present invention. The
scope of the present invention, therefore, is to be determined
solely by the following claims.
* * * * *